Conventionally, a single-component development method only using toner as developer and a two-component development method using toner and carrier are known as development methods for developing an electrostatic latent image formed on an image carrier in image forming apparatuses using electrophotographic methods.
In such a single-component development method, toner is commonly passed through a regulation section formed by a toner carrier and a regulation plate pressed against the toner carrier, thereby the toner is charged and a desired toner thin layer can be obtained, resulting in advantages in simplification, miniaturization, and cost reduction of an apparatus.
However, toner deterioration can be easily accelerated due to strong stress caused by such a regulation section, and the charge acceptance of toner can be easily decreased. Further, a regulation member as a charge providing member for toner and the surface of a toner carrier are contaminated with toner or external additives, whereby charge providing properties for the toner is decreased, whereby the charge amount on the toner is further decreased and problems such as fogging are caused. As a result, the service life of a development apparatus is usually shortened.
In contrast, in a two-component development method, toner is triboelectrically charged by being mixed with carrier, whereby causing small stress, and the carrier has a strong resistance to the contamination with toner or external additives, since the area of carrier surface is large.
However, in such a two-component development method, when an electrostatic latent image on an image carrier is developed, the image carrier surface is brushed with a magnetic brush formed of developer, resulting in such a problem that magnetic brush traces are generated in a developed image. Further, a carrier is easily allowed to adhere to the image carrier, resulting in the problem of image defects.
A so-called hybrid development method as a development method is proposed (refer to, for example, Unexamined Japanese Patent Application Publication No. H05-150636) to solve such an image defect problem and to realize high image quality comparable to that of a single-component development method while the service life is as long as a two-component development method using a two-component developer, in which hybrid development method a two-component developer is supported on a developer carrier and only toner is supplied from the two-component developer to a toner carrier for development.
However, in the hybrid development method of Unexamined Japanese Patent Application Publication No. H05-150636, there were problems such as decrease in density at a high development speed and development hysteresis (ghost)
The decrease in density at a high development speed is a problem where the flying of toner is not enough for a development nip time at a high speed image forming, thereby resulting in decrease in density.
The above problem is in common with noncontact single-component development. It has not been taken as a serious problem, since it has been used only in a slow speed region to avoid a problem of heat generation at a regulation section or a problem of toner fusion. In the hybrid development, these problems do not exist, whereby image formation can be carried out at a substantially high speed. However, for example, in an apparatus having a system speed of more than 500 mm/s, there is a possibility that the above problems are caused.
The problem of development hysteresis (ghost) is a commonly included in the hybrid development methods, and is a phenomenon where a post-development residual toner on a toner carrier which has not been used for development appears on a image as a development hysteresis (ghost) at the next development step.
In a facing portion (supply region) between the toner carrier and the developer carrier for supplying the toner carrier with toner, the toner is supplied, but the recovery of the post-development residual toner is conducted in the same facing portion. In the facing portion, a bias is applied in a such a direction that the toner is supplied in order to supply toner. This bias hinders the recovery of toner and the capability of recovering toner is not enough, whereby the difference in amount of residual toner between portions will appear as a contrast in density in the next development step.
As a countermeasure against the density decrease at high speed development, a method is proposed, in which a plurality of toner carriers are provided to lengthen the development time for toner flying to ensure toner density (for example, refer to Unexamined Japanese Patent Application Publication No. 2005-37523).
In the configuration of Unexamined Japanese Patent Application Publication No. 2005-37523, even when a photoreceptor is rotated at a high speed, owing to the plurality of toner carriers, toner can be flown more than once, whereby a toner image is surly formed on the photoreceptor, thereby reducing the density decrease of the toner image due to a higher speed. It is also disclosed that this configuration reduces the occurrence of ghost because a smaller amount of toner per a toner carrier is used for development in this case than in the case of only one toner carrier used for development, whereby the difference in density between the portions where the toner is used for development and the portions where the toner is not used for development is kept small.
However, in the hybrid development method, image forming highly depends on the distance between the image carrier and the toner carrier. Therefore, in order to obtain an appropriate image density of a formed image, needed is a configuration where the distance between the image carrier and each of the toner carriers is stably secured to be uniform in each axis direction.
In order to form an appropriate amount of toner thin layer on each of the toner carriers, magnetic poles must be provided in the developer carrier each to be face each of the toner carrier at an appropriate position
With the plurality of toner carrier provided, there is a high possibility of the errors to be high: the error of the position of the magnetic pole in the facing portion between each of the toner carriers and the developer carrier; and the error of the distance between each of the toner carriers and the developer carrier.
With the plurality of toner carrier provided, there may be a possibility of interference where the adjustment of position and distance for one of the magnetic poles causes error for other magnetic poles. The adjustment was difficult.